The challenge in this sorbent approach to hydrogen storage is in devising the appropriate functioning sorbent media. Much of the research on containing hydrogen in this way has focused on the property of various metals and metal alloys to reversibly chemically combine with hydrogen to form metal hydrides. Representative examples are FeTi; TiV alloys; LaNi5; various magnesium-nickel combinations; and sodium alanate, NaAlH4, which can reversibly dissociate in the presence of some catalysts into Al, NaH and H2. There is a substantial literature on metal hydrides and while research is still being actively pursued in this field for instance on tri- and multi-metal hydrides, demonstrated gravimetric hydrogen capacities are still well short of the 6+wt % hydrogen that are required for vehicular hydrogen storage.
There have recently been a number of claims of hydrogen storage by various forms of carbon: as single wall nanotubes by A. C. Dillon et al. in Nature 386, 377-379 (1997); and as graphitic nanofibers by Chambers et al. in J. Phys. Chem. B 102, 4253-4256 (1998). However, these and other literature claims of a substantial hydrogen containment by carbons [see A. C. Dillon and M. J. Heben in Appl. Phys. A 72, 133-142 (2001)] have not been specifically confirmed by other investigators; the field of hydrogen sorption by carbons has remained an open area of research. In this context, Cooper and Pez in U.S. Patent Application Publication No. 20020096048 have reported that intimate combinations of hydrogen reactive metals or metal alloys, or metal hydrides with various forms of substantially graphitic carbon, i.e. carbon-metal hybrids, display a reversible uptake of hydrogen at near ambient conditions and are useful as pressure-swing and temperature-swing sorbents for the storage of hydrogen. The observed reversible facile hydrogen reactivity is theorized to occur either by a “hydrogen-spillover” mechanism or by a partial reversible metal-catalyzed hydrogenation of the unsaturated graphitic carbon structures.
A recent report by S. J. Cho et al. in ACS Fuel Chemistry Division Preprints 47(2), 790-791 (2002) claims the adsorption of hydrogen by the hydrochloric acid-doped conducting polymers polyaniline and polypyrrole. These polymers are exposed to high pressure (1350 psia, 93 bar) hydrogen at 25° C. resulting in an apparent slow uptake of hydrogen gas. The hydrogen gas is desorbed by heating the sample to 200° C. at an unspecified gas pressure. The authors speculate that the hydrogen is physically adsorbed (i.e., the H—H bond remains intact) in the porous conducting polymers. Samples that were not treated with hydrochloric acid, which apparently induces porosity in the polymer samples, did not show any uptake of hydrogen. No metal catalysts are reported to be present in the material and no indication for chemical adsorption of hydrogen or catalytic hydrogenation of the polymer is given in this publication.
The possibility of storing hydrogen via the catalytic hydrogenation and then dehydrogenation of common aromatic molecules such as benzene or toluene has long been disclosed as a means of storing the hydrogen. With a theoretical hydrogen storage capacity of about 7 weight percent, the systems seem attractive. But while this chemistry is carried out routinely in chemical plants there are numerous difficulties in utilizing it in a practical hydrogen storage device.
The principal obstacles are as follows:
With the appropriate metal catalysts, the hydrogenation of benzene, toluene, naphthalene and related one or two six-membered ring aromatics to the corresponding saturated cyclic hydrocarbons, cyclohexane, methylcyclohexane and decalin, respectively, can be conducted at relatively mild conditions, e.g. ˜100° C. and ˜100 psi (6.9 bar) of hydrogen pressure, where it is thermodynamically very favorable. However, dehydrogenation of the above cited corresponding alkanes to produce hydrogen gas at the about 20 psia (1.5 bar) and higher delivery pressures that are required for use in fuel cells is, as currently carried out, is a highly endothermic process and thus requires the use of higher reaction temperatures than are not easily obtainable from fuel cells, especially those presently used in vehicles, as well as a significant input of energy. Thus, U.S. Pat. No. 4,567,033 to Kesten et al. describes a method of “freeing” molecular hydrogen from methylcyclohexane by its dehydrogenation to toluene at 316° C., the required thermal input being supplied by a combustion of a considerable portion of the by-product hydrogen.
Additionally, the common one or two six-membered ring aromatic molecules are quite volatile as are their hydrogenated products. While the hydrogenation can be conducted in a closed system, the production of product hydrogen from the reverse reaction fundamentally requires that there be some means of totally separating the gas from the reaction's organic volatile components. While technically possible, this requires a further unit operation which increases the complexity and hence the cost of the hydrogen storage process.
There have been several attempts to provide practical processes for storing hydrogen via a reversible hydrogenation of aromatics. U.S. Pat. No. 6,074,447 to Jensen et al. describes a means of dehydrogenating a hydrocarbon to an aromatic and hydrogen in the presence of a particular iridium-based molecular complex catalyst at preferably 190° C. or higher. Specifically described hydrocarbons are methylcyclohexane, decalin, dicyclohexyl, and cyclohexane (for which the corresponding aromatic products are toluene, naphthalene, biphenyl and benzene); there is no mention of any larger hydrogenated hydrocarbons or the polycyclic aromatic hydrocarbons or other pi-conjugated molecules of this invention. Additionally, the envisaged substrates of this prior art are clearly volatile at reaction temperatures and the reaction chamber is thus necessarily provided with a membrane that is highly selective for the passage of hydrogen as compared to the other volatile reaction components which are retained in the reaction chamber.
N. Kariya et al. have recently reported in Applied Catalysis A, 233, 91-102 (2002) what is described to be an efficient generation of hydrogen from liquid cycloalkanes such as cyclohexane, methylcyclohexane and decalin over platinum and other platinum-containing catalysts supported on carbon. The process is carried out at from about 200° C. to 400° C. under “wet-dry multiphase conditions”, which involves intermittently contacting the saturated liquid hydrocarbon with the heated solid catalyst in a way such that the catalyst is alternately wet and dry. Because of local superheating and other cited factors the dehydrogenation reaction is rendered more efficient in terms of improved reaction kinetics but because of the reaction thermodynamics (vide infra) it still requires the use of relatively high temperatures for a high conversion of the cyclohexane to the corresponding aromatic molecule. This basic process is elaborated on in several Japanese patent applications (e.g. JP20001110437 and JP2002134141) where it is applied citing benzene, toluene, xylene, mesitylene, naphthalene, anthracene, biphenyl, phenanthrene and their alkyl derivatives as possible aromatic substrates as a means of producing hydrogen for fuel cells. It is evident however, that for this and other implementations of the process, active means for totally separating the product hydrogen from the volatile components of the process need to be employed.
R. O. Loufty and E. M. Vekster, in “Investigation of Hydrogen Storage in Liquid Organic Hydrides”, Proceedings of the International Hydrogen Energy Forum 2000, Munich Germany, 2000; pp. 335-340, have reported the dehydrogenation of decalin in a membrane reactor where the very low conversion (˜15%) of decalin, even at 300° C., is greatly enhanced by the selective separation of hydrogen by the membrane and its removal from the reactor.
JP2002134141 A describes “liquid hydrides” based on phenyl-substituted silanes; aryl-substituted oligomers and low molecular weight polymers of ethylene; low molecular weight polymers of phenylene; and oligomers of aryl- and vinyl-substituted siloxanes where the aryl groups are phenyl, tolyl, naphthyl and anthracyl group.
In spite of the work described above, there remains a need for processes for the reversible hydrogenation of pi-conjugated substrates to provide for the storage and release of hydrogen at practical operating temperatures and pressures, particularly for supplying hydrogen to fuel cells.
Citation of any reference in Section 2 is not an admission that the reference is prior art to the present application.